Industrial controllers are special purpose computers used for controlling industrial processes or manufacturing equipment.
Under the direction of a stored program, the industrial controller examines a series of inputs reflecting the status of a controlled process or controlled equipment and changes outputs affecting control of the process or equipment. The inputs and outputs are mostly binary, that is "ON" or "OFF"; however, analog inputs and outputs taking on a continuous range of values and multi-bit digital values, are also used.
Industrial controllers are frequently assembled from individual functional modules that fit within a rack to be joined together by means of a common backplane. A circuit card within the functional module has conducting fingers to engage a multi-line connector attached to the backplane and the backplane supplies power to the functional modules and a number of parallel conductors for data communication between the functional modules.
By dividing the operation of the industrial controller among the functional modules, the industrial controller may be flexibly configured to meet a given controlled task. For example the processing unit of the industrial processor may be one circuit card and separate from the input and output circuits for receiving data from and transmitting data to the industrial process. If additional I/O lines are needed, additional I/O cards may be inserted within the rack.
The modularity of the industrial controller also improves its serviceability. A failure in a given component may be isolated by "swapping" circuit cards in and out of the rack to see if the problem is remedied. In most cases only a single card will have faulted and that card alone may be replaced, reducing the cost of the repair.
Ideally, the removal and replacement of the circuit cards of the industrial controller could be done while the industrial controller is operating under power. The capability for removal and insertion under power (RIUP) decreases the time necessary to isolate a given fault, and in some cases permits some portions of the industrial process to continue to operate while a repair is being undertaken.
Permitting the removal or insertion of circuit cards while the industrial controller is under power is not a simple matter. First, the circuit cards normally contain an electronic processor and an associated, volatile electronic memory. Removal of the power to such a memory erases its contents, rendering the board inoperable until the memory is reprogrammed, making board swapping impractical. One method of solving this problem is to provide an on-board backup battery for the memory.
One problem with this approach of using a backup battery is that in order to conserve battery power the remainder of the devices on the board, including those connected to the memory, are allowed to lose power. The outputs of these connected devices may move to undefined logic levels as power drops. An undefined logic level is a voltage that is not high enough to be reliably interpreted as a logical true nor low enough to be reliably interpreted as a logical zero. These undefined logic levels can cause an unintended writing to the electronic memory of spurious data or can cause the electronic memory to enter a state of extremely high current drain from the backup battery. This high drain current can lower the battery voltage to the point where the electronic memory loses its stored information.
One method of avoiding this problem is to install a set of electronic switches between the volatile electronic memory and the devices providing outputs to that memory (typically the electronic processor) to completely disconnect the processor from the memory when a power loss is detected.
Nevertheless, while this method prevents high current drain or unintentional writing to the electronic memory during power failure, the electronic switches introduce a transmission delay in signals passing through the switches (a gate delay) slowing the speed of reading or writing the memory and thus slowing the operation of the functional module.
Providing additional battery backup to the processor, besides unduly increasing the battery drain, can provide problems with the input protection circuitry of other devices connected to the electronic processor, particularly with processor output signals that are de-asserted at a logical high level. Referring to FIG. 11, other devices such as EPROM or PROM, connected to the electronic memory but not provided with battery backup, have diodes 124 and 125 connected between their inputs 120 and ground and V.sub.cc. These diodes are oriented to safely conduct out-of-range voltages away from the input by conducting voltages higher than V.sub.cc to V.sub.cc and lower than ground to ground. As V.sub.cc drops however, a current may flow from the de-asserted lines of the processor providing the input 120 (supported by the battery backup voltage) through the diodes 124 to the falling V.sub.cc. Thus, if the voltage on input 120 exceeds V.sub.cc, current will flow through protection diodes 124.
A second obstacle to the removal and insertion of circuit cards while the industrial controller is under power is electrical arcing that may damage the connector attaching the circuit card to the backplane. Normally the circuit card will have a number of filter capacitors which present a very low impedance to the power supply in the backplane when the circuit card is first installed. The resulting high inrush of current may etch or pit the electrical connector joining the circuit card to the backplane reducing its reliability.
Finally, this large inrush of current may produce a momentary decrease in the regulated power supply feeding another circuit card thus adversely affecting the operation of the other card. One method of eliminating the latter problem of high momentary loading of the backplane power supply is shown in FIG. 10. Here the connector 100 joining the circuit card 103 to the backplane 20 employs long connector pins 104 and short connector pins 106 on one-half of the connector 100 which connect with equal height sockets 108 on the other half of the connector 100. The long connector pins 104 provide electrical connection to corresponding sockets 108 before the short connector pins 106 provide such connection as the circuit card 103 is inserted into the backplane 20.
During an insertion of the circuit card 103, the long pins 104 first connect the circuit card 103 to a precharging power supply 102 that is isolated from a main operating supply 105 used by the circuit card 103 during normal operation. The precharging power supply 102 charges the filter capacitances associated with the card 103. Moments later, the short pins 106 connect with the main operating supply 105 after the filter capacitance 114 has been fully charged. Thus, there is no disturbance to the main power supply 105.
When another circuit card is inserted, the precharging supply 102 may experience a downward voltage spike caused by the low impedance of the charging capacitance 114. This drop in voltage is isolated from the other circuit cards firmly connected to the main power supply 105 by diodes 116 which prevent current flow from the main power supply 105 to the precharging power supply 102. The precharging power supply 102 operates at a higher voltage to compensate for the voltage drop across diodes 116.
Nevertheless, this precharging technique also has drawbacks. First, the long pins 104 still experience large charging currents which may cause pitting to those pins. Second, if the circuit card 103 is inserted too quickly, the long pins 104 will not be connected for sufficient time to fully charge the capacitance 114. Third, if the circuit card 103 is inserted too slowly or not fully seated, the circuit card 103 may begin operating on the precharging power supply instead of the main power supply 105 and thus be susceptible to disturbances in that supply caused by the insertion of other cards. The precharging power supply 102 is supplied by relatively small traces to allow more backplane copper for the main power supply 105 and hence experiences greater voltage drops with changes in current demanded. Fourth, the precharging technique requires redundant backplane traces, connector pins and power supplies.